This invention relates to an exhaust gas recirculation method for internal combustion engines for automotive vehicles, and more particularly to a method of this kind which is applied when the vehicle is in a starting condition.
An exhaust gas recirculation method, i.e. so-called EGR, which aims at reducing nitrogen oxides (NOx), emitted from an internal combustion engine for automotive vehicles, by returning part of exhaust gases from the engine to an intake passage thereof is known e.g. from Japanese Provisional Patent Publication (Kokai) No. 57-188753 which comprises previously setting required valve opening values of an exhaust gas recirculating valve arranged across an exhaust gas recirculating passage communicating between an exhaust passage and the intake passage, and storing the set required valve opening values in storage means, reading one of the stored values from the storage means in response to values of a plurality of operating parameters of the engine (e.g. intake pipe absolute pressure at a zone downstream of a throttle valve in the intake passage and rotational speed of the engine) so as to obtain an optimal exhaust gas recirculation amount, and controlling the valve opening of the exhaust gas recirculating valve to a desired valve opening corresponding to the read required valve opening value.
According to the conventional exhaust gas recirculation methods, the required valve opening values of the exhaust gas recirculating valve are set in such a manner that the amount of recirculated exhaust gases is increased during acceleration of the engine when NOx is emitted in large quantities, whereas during deceleration the amount of recirculated exhaust gases is decreased to thereby secure stability in combustion.
In this kind of conventional control method, the engine is determined to be accelerating when increases are detected in the engine rotational speed and the intake passage absolute pressure are detected. As a result, when the vehicle is in a starting condition where the engine rotational speed and the intake passage absolute pressure suddenly increase, the engine is erroneously detected to be accelerating, which entails an increase in the exhaust gas recirculation amount. However, at vehicle start when high output is required of the engine, increased recirculation of exhaust gases tends to deteriorate the combustion performance of the engine whereby the engine fails to produce sufficient output to start the vehicle and becomes liable to undergo stalling, etc.
One way of solving this problem is to equip the engine with detecting means such as gear shift position detecting means and clutch engagement detecting means for detection of vehicle start, whereby when the vehicle is detected by the detecting means to be starting the exhaust gas recirculation amount is decreased. However, the provision of such detecting means results in a more complicated construction of the exhaust gas recirculation control system as well as higher production cost of the control system.